Reference design non-isolated LED driver Introduction

"This application note presents a reference design for a non isolated LED driver designed to operate directly from a 400VDC input. The design drives a string of 27 wleds (white LEDs) or optional 6 AMBER LEDs at 400mA. The topology is discontinuous flyback with transformer. With max16801 HB (high brightness) led controller. Brief circuit description The reference design is a flyback LED driver for offline environment (400VDC). The design can drive 27 wleds (white LEDs) at 400mA. With jumper J1 installed, the design drives six amber LEDs at 400mA. The design uses a max16801 HB LED controller and a three winding transformer (coupled inductor). Since the current sensing is fed directly into the IC control loop, there is no electrical isolation. The turns ratio of the transformer is 18:6:1. The primary inductance is 800 µ h, the rated current is 750ma (peak), and the duty cycle is always less than 50%. The operating frequency is 265khz and is not adjustable. Overvoltage protection (non latching) is 120V. UV detection level is 310V. The on delay time is about 43 milliseconds, after which the VIN is about 22V, and the IC will start driving the external MOSFET. In turn, this will cause the VIN capacitor to decay until the bootstrap winding can provide support. Due to the high impedance of the LED at low voltage, the main secondary load will initially be only the output capacitor. The ratio of secondary to tertiary turns is 6:1, which means that once 60V voltage is formed at both ends of the output capacitor, the bootstrap winding will provide 10V voltage to the IC. For the 6 LED string option (i.e. J1 is installed), the output capacitor must obviously be charged to 10.7v before the IC can use 10V voltage. The calculated peak current of the inductor primary winding is 750ma. By clamping the secondary winding in the middle of the shunt primary, the leakage inductance can be minimized. The measured primary leakage inductance is less than 5 µ H. Since this value is very low, there is no special measure to dissipate the leakage inductance energy. All leakage energy dissipates in the MOSFET itself. The temperature rise of the transformer is less than 30 ° C. The switch MOSFET has a spacer to ground the radiator. This minimizes the metal surface area experiencing high-speed voltage transients, thereby minimizing radiated EMI. The temperature of MOSFET rises less than 40 ° C. Editor: hfy, read the full text“

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